GiGi (aka GG) is a GUI library for OpenGL. It is platform-independent (it runs at least on Linux and Windows, and probably more), compiler-independent (it compiles under at GCC 3.4 or higher and MSVC++ 8.0 SP1 or higher, and probably more), and driver-independent. Reference drivers for SDL and Ogre are provided, and it is straightforward to write one for yourself should you decide to do so.